Analysts at the firm Wood Mackenzie estimated the field produced more than 70 percent of the billion barrels over the past two years. According to its website, the field now accounts for about 16 percent of the oil produced daily in the United States.

More than half of the field's production comes from 10 percent of its 20,000 square miles, the firm said.

Those billion barrels have been produced since 2008. The firm says that makes the field the third-fastest to have produced a billion barrels, trailing only the Alaska North Slope and the Ghawar field in Saudi Arabia, and more than 10,000 wells have been completed in the field.

"It puts the Eagle Ford in elite company on the world scale. It makes it even more clear that this is a world-class play," Wood Mackenzie analyst Cody Rice told the San Antonio Express-News.

Looking ahead, the firm projects the field will produce the equivalent of 2.8 million barrels of oil per day, or 42 billion gallons, in 2015. It also projects a 2015 capital expenditure of $30.8 billion in the Eagle Ford Shale, almost twice the next-biggest expenditure, $16.7 billion in the Bakken field in North Dakota and Montana.